Evolution of Sex and the DNA repair hypothesis

DNA can be damaged in at least two ways. First, ionizing radiation or mutagenic chemicals can alter the genetic code. Second, mutations can occur via errors during the replication process. Most mutations are deleterious. In asexual organisms, any mutation that occurs in one generation will be passed on to the subsequent generation. Consider photocopying a document, then photocopying the photocopy, then photocopying the photocopy of the photocopy and so on. Clearly, the quality of the document deteriorates the more copies that are produced. Likewise, asexual organisms -- as they continue to accumulate mutations -- face the prospect of eventually becoming both unable to reproduce and unviable -- neither of which is at all helpful to evolution.

But sexual reproduction allows most plants and animals to create offspring with good copies of two genes via crossover and would thus help eliminate this quality depletion since mutations -- although they might still be passed on from one generation to the next -- would not necessarily be expressed in the next generation, for a mutation must appear in the genes of both parents before it can be expressed in the offspring.

Evolution of Sex and the DNA repair hypothesis - A Conclusion
One must not overlook an important fact in this theory. The theory attempts to explain why sex exists now, but does not address the origin of sex. It is one thing to develop a theory or hypothesis to explain something that already exists, but it is entirely another to develop a theory to explain how that specific phenomenon came into being.